Virtual Destructors

A problem can occur when using polymorphism to process dynamically allocated objects of a class hierarchy. So far you have seen nonvirtual destructorsdestructors that are not declared with keyword virtual. If a derived-class object with a nonvirtual destructor is destroyed explicitly by applying the delete operator to a base-class pointer to the object, the C++ standard specifies that the behavior is undefined.

The simple solution to this problem is to create a virtual destructor (i.e., a destructor that is declared with keyword virtual) in the base class. This makes all derived-class destructors virtual even though they do not have the same name as the base-class destructor. Now, if an object in the hierarchy is destroyed explicitly by applying the delete operator to a base-class pointer, the destructor for the appropriate class is called based on the object to which the base-class pointer points. Remember, when a derived-class object is destroyed, the base-class part of the derived-class object is also destroyed, so it is important for the destructors of both the derived class and base class to execute. The base-class destructor automatically executes after the derived-class destructor.

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Good Programming Practice 13.2

If a class has virtual functions, provide a virtual destructor, even if one is not required for the class. Classes derived from this class may contain destructors that must be called properly.

Common Programming Error 13.5

Constructors cannot be virtual. Declaring a constructor virtual is a compilation error.